The invention concerns an improved device to test the permeability to air of articles, called "test objects", in particular felts and screens used in machines and installations for manufacturing paper.
A prior art device of this type is known from EP-Pat. No. 0 096 31. This prior art device is designed as a movable instrument whose measuring channel is separated into two coaxial channel sections that define between each other a measuring aperture. The test object, for instance, a sheet made of fabric, a screen, or a felt, must be inserted into the measuring aperture and, subsequently, must be clamped tight. This prior art device is a technically complex construction which must be attached to a multitude of positions on the test object in order to arrive at permeability profiles, a procedure that takes a correspondingly long time. The test object must be clamped tight in order to obtain as uniform an air flow through the test object as possible and to avoid significant losses through leakage. By basing the design on a belt channel with equal pressure it is further possible to avoid most leakage losses that are caused by the surface structure of the test objects.
The purpose of the invention is to present an improved test device with the properties defined by the general characteristics of claim 1 that permit the determination of the permeability to air at different areas of a test object, rapidly and in a simple fashion, combined with the advantages of encompassing a wide range of values, covering at least a ratio of 50, and with high accuracy. The test device is usable with ease and without problems, it is appropriate for a large variety of objects having different permeability, and it is locally movable in order to allow for the acquisition of different permeability profiles. Finally, the device is applicable even in the case of slowly moving test objects.
The device permits the measurement of air permeabilities without requiring the test object to be clamped between the segments of the channel in which the air flow is measured. Instead, the invention makes it possible to attach channel segments to a single side of the test object, a procedure that has the advantage in that the device can be used for test objects which are only accessible from one side and/or which move continuously.
The test device can be constructed in such a fashion that it has a low weight and can be used manually with ease. This feature has the advantage that the test device allows for continuous measurements while it is being moved relative to a test object that itself is in motion. As a consequence, longitudinal and transverse measurements can be made.
In the process, the pressure drop in the case of diverse fabric permeabilities is automatically set to a predetermined value and kept at this mark. The replica of the natural flow through the test object, due to the screening of the flow in the measuring channel by the test object, by the air flow in the belt channel has the advantage to protect the air flow in the measuring channel from exterior influences and to make available an unavoidable amount of leakage air.
Thus, the test device permits the determination of air permeabilities for felts, screens, fabrics, and similar sheets in a short time. The permeability properties have a significant influence on various operating conditions of machines that are run with such felts or screens.
These and other objects of the invention will become apparent in light of the present specification and drawings.